This invention relates generally to a molding method and apparatus and more particularly to a method and apparatus for molding a product having internal threads. More particularly, the invention relates to withdrawal of a threaded mold core from the molded product. The invention is especially useful for manufacturing products made from polymers.
The injection molding of a product using polymers or like material, requires a mold having an inverse image of each surface of the product. For internal cavities, it is necessary to provide a mold core that is separate from the mold and is withdrawn in a direction different from the direction of separation of the mold halves. When the internal cavity includes a threaded surface, the mold core cannot be simply axially withdrawn because such would interfere with the newly formed threads. Nor can the newly formed threads serve as a guide for threadably backing-out the core because such newly formed threads do not have sufficient strength at the time that the mold core must be removed.
One solution has been to support the mold core with a metal lead screw and nut having mating threads which correspond to the pitch of the molded threads in order to carry all of the load of withdrawal of the core and thereby avoid deforming of the plastic threads. The problem with this approach to core withdrawal is that an equal amount of time is required to reset the mold core by reversing rotation of the drive motor to re-thread the lead screw in the nut. Another problem proper is repositioning of the core. Because the core must be precisely repositioned against the mold, a low-fluid pressure source must be applied to the drive motor in order to avoid damage to the mold or corefrom excess torque. However, the low-fluid pressure is insufficient to break loose the core from the newly formed threads, so the drive motor must be switched between high and low pressure sources.
In order to overcome this difficulty, it has been proposed to mount the mold core on a carrier and to control the movement of the carrier with a lead screw, which is rotated concurrently with the core. The core is reset by forwarding the carrier against a fixed stop. This overcomes the necessity of providing a low pressure source for resetting of the core and allows for a reduction in components in order to service an injection molding apparatus having multiple cores. However, several problems remain. The time duration to reset the core withdrawal mechanism is likewise as lengthy as the time to actually withdraw the core because the lead screw must be rotated in reverse to reset the carrier. Importantly, mold changeover for a different product is exceptionally time-consuming and requires a large stock of components on-hand in order to accommodate various thread pitches.
Conventional injection molding machines have employed a control theory based on the assumption that the steps performed in the process of bringing together the mold components in order to form the product will be carried out in reverse sequence when disassembling the mold components in order to remove the final product from the mold. Therefore, controls on existing plastic molding machine designs, reinforce the necessity for reinserting a threaded mold core in a manner that is a precise mirror image of the method of withdrawing the mold core. Any deviation from this control philosophy would be considered incompatible with existing plastic molding machines, and hence, impractical because retrofit of existing molding machines would be prohibited.